Although active noise cancellation systems are well known for use with the ventilation ducts of buildings, such systems have not been found to be readily applicable to noise reduction as a substitute for passive mufflers in motor vehicles. In addition to packaging problems relating to acoustical coupling between the transducers and the high temperature exhaust conduit, exposure to harsh environmental conditions and vulnerability to contact with foreign objects, these problems must be addressed economically so that the noise cancellation system can be implemented in a mass production process without substantially increasing the cost of manufacturing or installing the components.
U.S. Pat. No. 4,473,906 to Wanaka et al discloses numerous prior art sound attenuation system embodiments. The patent discusses the inclusion of additional transducers and electronic controls to improve the performance of the active acoustic attenuator, by reducing the effect of the feedback of the cancellation signal which arrives at the sensor.
U.S. Pat. No. 4,677,677 to Erickson further improves attenuation by including an adaptive filter with on-line modeling of the error path and the cancelling speaker by using a recursive algorithm without dedicated off-line pretraining. U.S. Pat. No. 4,677,676 adds a low amplitude, uncorrelated random noise source to a system to improve performance. Likewise, U.S. Pat. Nos. 4,876,722 to Decker et al and 4,783,817 to Hamada et al disclose particular component locations which affect performance. These patents do not teach or suggest the adaption of active attenuator noise control systems to motor vehicles.
It will be appreciated that the sound pressure pulses emitted from the transducer must be of sufficient power to cancel out the sound pressure pulses travelling through the exhaust conduit. In order to initiate the high energy pulses required to cancel the high level of sound emanating from the motor vehicle engine, a relatively large amplifier is required to drive the transducer. In addition, the transducer must be capable of withstanding the power provided by the amplifier. Moreover, the electro mechanical transducer and the power amplifier are substantially higher cost hardware items than the other components of the noise attenuation system. Accordingly, the power requirements for a system intended to be used as a motor vehicle muffler directly conflict with the lack of packaging space and the need for minimizing production costs of motor vehicles, and represents a substantial impediment to the incorporation of such systems in a motor vehicle.